This invention relates, in general, to an apparatus and method for thermally treating a semiconductor substrate, and more particularly to an apparatus and method for RTR recrystallization of semiconductor substrates.
In the manufacture of semiconductor devices such as transistors, integrated circuits, photovoltaic devices, and the like, the semiconductor industry uses large quantities of semiconductor material, usually silicon, in the form of thin wafers or sheets. It has been conventional to produce the semiconductor wafers by first growing a single crystal semiconductor ingot, sawing the ingot into a plurality of thin slices, and then lapping and polishing the slices to the desired thickness and surface finish. While this process has proved satisfactory for most semiconductor devices, it is too expensive for some large area semiconductor devices and especially for large area photovoltaic devices or solar cells. In fact, in order that photovoltaic devices become a viable alternative energy source, a significant reduction in the cost of the semiconductor starting material is essential.
One technique which has been proposed and developed for the production of thin sheets of semiconductor material suitable for the production of solar cells is the so-called ribbon-to-ribbon (RTR) conversion process. In this process a polycrystalline ribbon is transformed directly into a macrocrystalline ribbon without the need for costly processing of large diameter ingots. The RTR process uses one or more scanned beams of energy impinging on a polycrystalline ribbon to locally melt the ribbon and to induce crystal growth as the ribbon is translated past the energy beam. As the molten zone moves along the ribbon, the material behind the zone resolidifies in a macrocrystalline form. The macrocrystalline structure is one in which the crystals are of sufficiently large size to permit efficient semiconductor action. Therefore, a monocrystalline ribbon wherein the ribbon is but a single crystal is encompassed within the term "macrocrystalline." In this context, the word "ribbon" generally implies an elongate strip or sheet having a width much greater than its thickness. Typical dimensions might be a length of 15-30 cm, a width of 1-10 cm, and a thickness of 50-250 micrometers.
In the conventional RTR process the ribbon is preheated to a temperature approaching its melting temperature by moving the ribbon lengthwise through an increasing temperature. This, of course, imposes a temperature gradient along the ribbon from end to end. After melting, the ribbon again moves lengthwise, this time through a decreasing temperature. Imposing a temperature gradient along the ribbon tends to stress the ribbon resulting in bending, buckling, or shattering. The closer the temperature profile is to a linear profile, however, the less the stress. It is also believed that non-constant thermal gradients contribute significantly to the formation of dislocations which degrade minority carrier diffusion length and hence solar cell efficiency.
The need to raise the temperature of the ribbon to a temperature approaching its melting point and the deleterious effects of non-constant temperature gradient on ribbon morphology make the control of temperature gradient an important and difficult task in RTR recrystallization. The need to control temperature gradient has an adverse effect on equipment complexity and expense, on the quality of macrocrystalline ribbon produced, and on the speed and cost with which the ribbon can be produced.
While the effect of thermal gradients is important in RTR processing, the effect is also important in a host of other thermal treatments routinely encountered in semiconductor processing. Thermal gradient stressing is encountered, for example, in high temperature impurity diffusion into a semiconductor wafer. In that process a plurality of semiconductor wafers are typically loaded into a wafer holding boat and the boat is pushed into a furnace heated to 900.degree. C. or more. In so heating the wafers, nonlinear temperature profiles are established because of a nonisotropic thermal environment caused by the proximity of adjacent wafers and the presence of the non-uniformly distributed thermal mass of the boat. The thermal stress induced by nonuniformly heating and cooling the wafers leads to the generation of dislocations and the reduction in minority carrier lifetime.
To produce improved devices, therefore, it is desirable to avoid defects induced while thermally treating a semiconductor substrate. Accordingly, it is an object of this invention to provide an improved process for thermally treating semiconductor substrates.
It is another object of this invention to provide an improved process for RTR recrystallization of semiconductor substrates.
It is another object of this invention to provide improved apparatus for thermally treating semiconductor substrates.
It is yet another object of this invention to provide an improved process to minimize thermally induced defects during a high temperature thermal treatment.
It is still another object of this invention to provide an improved process for rapid, high quality RTR processing of semiconductor substrates.